Using procedures developed in this laboratory for the acquistion and analysis of more data than is normally processed for spectral potentiometric studies, we have completed the analysis of cytochrome aa-3 in intact mitochondria and in isolation. Our analyses support the following unique new characteristics for the enzyme. (1) There are three Nernstian components. (2) The Em values are 200 mV, 260 mV, and 340 mV with corresponding n values of 2, 2, and l. (3) The individual absorption maxima for the Soret and Alpha peaks are 429 and 602 nm; 446, and 605 nm; and 448 and 607 nm, respectively. (4) Cytochrome a-3 is the component with Soret maximum of 429 nm and Aplpha at 602 nm. (5) The redox potential of heme a-3 is under the control of the redox state of heme a; it is 200 mV when heme a is reduced and greater than 420 mV when heme a is oxidized. (6) The Em of heme a-3 is not raised in an atmosphere of carbon monoxide and this behavior indicates that protons are held less strongly by the reduced liganded enzyme than the oxidized form. The general result of our studies with all of the mitochondrial cytochromes is that the electron transport chain passes 2 electrons at a time rather than one. This conclusion is consistent with a number of other observations. We have analyzed earlier methods of data acquistion and analysis and have found that they are not capable of resolving the mixture and character of cytochromes revealed by our methods.